An exhaust catalytic converter only performs its task of reducing the unburnt hydrocarbons, carbon monoxide and oxides of nitrogen content of the exhaust gases after it has reached a critical temperature, termed the light-off temperature, which is between 300.degree. C. and 400.degree. C. During cold starts, it is important to minimise the time taken for the catalyst to reach this temperature, more especially since the emission test drive cycles which are laid down by various legislations all include a cold start.
Various solutions have already been put forward to enable the light-off time to be reduced. The simplest solution is to place the catalyst very near the engine so that it is heated by the exhaust gases before these have been cooled by the exhaust system. This method of mounting a catalyst, usually termed close-coupling, creates problems when the engine is running under high speed and high load conditions. Under such conditions, the exhaust gas temperature can exceed 850.degree. C., which is enough to cause permanent damage to the catalyst. It is therefore preferred not to provide a close-coupled catalyst but to use one mounted some distance away from the engine, normally termed an under-body catalyst. Such mounting is safe for high speed and high load operation but exacerbates the warm-up problem because the exhaust gases are cooled before reaching the catalyst during the start-up phase.
To speed up the warming of a catalytic converter, an external heat supply has been proposed, including electric heaters and microwave heaters. These proposals have involved significant additional cost and complexity, more especially when it is appreciated that the power requirement is of the order of 2 to 3 kilowatts, which with a 12 volts supply calls for a current of 166 to 250 amps.
It has also been proposed to use chemical energy to reduce light-off time by injecting fuel into the exhaust pipe and igniting it. The complexity in this case is that petrol/air mixtures do not always ignite reliably when diluted with exhaust gases from the engine and if they should fail to do so they aggravate the problem by cooling the catalyst and by dramatically increasing the hydrocarbon emissions in the exhaust. There are further complexities imposed by the need to ensure safety, it being inherently dangerous to provide a fuel line opening into a hot exhaust pipe.
A still further proposal has been the use of the so-called thermal reactor in which air is injected into the exhaust stream close to the exhaust port to intercept the exhaust gases while they are still hot. If the mixture is set slightly rich, the combustion reaction continues in the exhaust gases, albeit at a reduce rate, and this raises the temperature of the exhaust system to reduce the light-off time of the catalytic converter. Though this proposal works, the benefits one achieves by it are only of limited value. Typically, the light-off time would be reduced to around two minutes, which still falls short of enabling the more exacting permitted legal emission levels to be met.
A still further proposal has been to use an afterburner. The engine is once again run with a rich mixture and fresh air is added to the exhaust gas stream but this time the mixture is ignited, for example by a spark to burn within a chamber arranged immediately upstream of the converter.
It is important to differentiate between the reaction initiated by ignition in an afterburner and the reaction which normally takes place on the surface of a catalytic converter. In an afterburner, there is created a luminous open flame which propagates through the gases and is not confined to a surface. The ignition can be initiated by a spark, a pilot flame or indeed by a heated catalytic element. Once ignited the flame is not confined to the igniter and the gases burn as they would in an unconfined space.
The concept of an afterburner is not in itself new and it has been known since 1967 that under controlled conditions one can re-ignite the fuel in the exhaust mixture. In a report by C. D. Haynes published by the Motor Industry Research Association of Great Britain (MIRA) as report No. 1967/5, there is an early suggestion to use an afterburner as a means of reducing pollution, the heat it produces being merely dissipated in a heat sink. The heat sink can of course be the matrix of a catalytic converter so that afterburner may act to reduce the light-off time of the converter.
The use of an afterburner to heat exhaust gases before they reach a catalytic converter has been specifically suggested in U.S. Pat. No. 3,889,464 in which patent the fuel for the afterburner is not derived from the exhaust gases. A development of this idea described in EP A 0 422 432 uses the partly burnt combustion products in the exhaust gases to fuel the afterburner. In the latter proposal, the mixture strength to the engine is enriched by diverting some of the metered air to flow directly into the exhaust pipe.
For the purpose of heating the catalytic converter to reduce its light-off time, the afterburner has been the most effective proposal to date. When the engine is run with a moderately rich mixture and fresh air is added to the hot exhaust gases after the exhaust system has warmed up, it is possible to re-ignite the mixture because a so-called cool flame reaction is still taking place in the exhaust system. This allows the warm-up time to be reduced to less than one minute.
In the prior art proposals, however, one must wait some time after the engine has started before the gases become ignitable in the afterburner. This is because when the engine and the exhaust system are cold, the mixture arriving at the afterburner will have lost most of its heat to the exhaust system and any cool flame reaction taking place in the gases as they left the engine will have been quenched during passage through the cold exhaust manifold and down pipe and will have been quenched further upon injection of the additional cold air into the exhaust gas stream. In the absence of the cool flame reaction which is known to assist ignition, the exhaust/air mixture will not be ignitable. One must wait until the exhaust pipe has warmed to a temperature which enables the cool flame reaction to be sustained until the exhaust gases reach the afterburner. Once the afterburner has fired, the catalytic converter will be heated rapidly to its light-off temperature but during the initial phase before the afterburner is ignited, the exhaust gases are discharged to atmosphere without being cleaned neither by the afterburner nor by the catalytic converter.